The markets are acting extremely skittish in after-hours trading, on the heels of a pair of bombs exploding near the finish line of the Boston Marathon on Monday afternoon, killing two and injuring at least 23 people. With approximately 30 minutes left in trading, two blasts within seconds of each other were heard slightly more than four hours into the renowned distance race, roughly two hours after the race’s top finishers crossed the finish line.

Prior to reports of the explosions hitting the wires, the S&P 500 was already headed for its worst day in more than four months, as big drops in the price of gold and other commodities triggered a broad selloff in equities. All three major U.S. stock indices extended declines in the last 30 minutes following the reports of the explosions, and we are continuing to see major weakness in after-hours, setting the stage for a volatile trading day tomorrow.

On the day, the S&P 500 lost 36.49 points or -2.30% to finish at its lows of 1,552.36. The slide is continuing in after-hours, with losses extending to -2.50%. Traders will be closely watching the 1,540 level on the index tomorrow, as this provided support in the 1,540-1,570 range that we saw throughout most of March. Additionally, a breach below this level will likely be met with additional pressure, as the index is in jeopardy of falling below its 50 day-moving average for the first time since December.